The quake hit around 7:20am Tuesday (21:20 GMT Monday) 33 kilometres southeast of Bulolo, on the country's eastern side, at a depth of 127km. It was felt in the capital Port Moresby about 250km away.
The Pacific Tsunami Warning Center said there is no tsunami threat because the earthquake was so deep.
"We have no reports as yet" of serious damage, Inspector Leo Kaikas, Bulolo police station commander, told the AFP news agency. "We are still assessing the situation."
Staff at Bulolo's Pine Lodge hotel said there was very minor damage from objects falling off tables, but nothing more serious.
Residents in Lae, more than 100km away, said the earthquake knocked things off shelves and worktops and cut electricity in some areas.
"I had just woken up," said Lae resident Christopher Lam. "It lasted a little more than 30 seconds. We had household items knocked off their shelves and the power got cut.
"Things seem to have returned to normal. No structural damage here, though I'm not sure about other buildings in the city."
There are estimated to be around 110,000 people living within 50km of the epicentre, according to UN data.
The Moresby-based National Disaster Management office said while there were no early reports of damage, but news from the quake zone could take time to trickle in.
The country's rugged highlands region was hit by a 7.5-magnitude quake in February last year that buried homes and triggered landslides, killing at least 125 people.
The scale of that disaster did not become apparent for days due to PNG's poor communications and infrastructure.
There are regular earthquakes in Papua New Guinea, which sits on the so-called "Pacific Ring of Fire" – a hotspot for seismic activity due to friction between tectonic plates.
Along the South Solomon trench, an area of the Pacific that includes PNG, there have been 13 quakes of magnitude 7.5 or more recorded since 1900, according to USGS data.
SOURCE: News agencies